Spark plugs usually have a tubular metal housing which is externally threaded to be fitted into the cylinder head of an internal combustion (IC) engine. A rotation-symmetrical insulating body, which is highly heat-resistant, is located within the metal housing. The metal housing, normally, has a central opening or bore in which the insulator is fitted. The insulator, typically, is a ceramic, which may have a porcelain glaze on the outside. The spark plug has a terminal end, outside of the combustion chamber, adapted to receive an electrical terminal connected to an ignition cable; and a combustion end, in which a center electrode, passed through the insulator, projects from the insulator by a small distance, to form a spark gap with one or more ground or chassis electrodes which protrude from the combustion chamber side of the metal housing.
It has already been proposed to form the center electrode of several elements, for example a terminal end which is positioned opposite the ground or chassis electrode to form therewith a spark gap, electrically in series with a metal core within the insulator, which projects outside to be connected to the ignition cable. It has also been proposed - see U.S. Pat. No. 2,603,200 - to retain an electrically conductive liquid within the central bore of the insulator, for example mercury, or a metal which has a low melting point, such as bismuth, tin, lead, or antimony. At normal operating temperatures, the liquid or readily liquefying metal, is in the liquid phase. Such spark plugs require a considerable time before the insulator at the ignition end reaches a temperature of between 400.degree. C. and 450.degree. C. This is the temperature at which deposits which collect on the insulator will burn off, and thus prevent leakage or creep or short-circuiting paths along the insulator between the center electrode and the metal housing. The heat transfer from the insulator to the liquid or readily liquefying metal within the center bore of the insulator is excellent during the initial heating of the spark plug, so that the heat transfer from the insulator to the metal body is good, and thereby extending the heating of the spark plug to the temperature at which deposits will burn off. Spark plugs of this kind, that is, which remain below the self-burning temperature of between 400.degree. C. and 450.degree. C., for a relatively long time are subject to deposits of electrically conductive materials, such as carbon black or soot, on the insulator body. The relatively long time before such deposits will burn off may lead to misfires of the spark plug, and electrical shunt paths. Such spark plugs usually have a thin central opening between the region where the liquid metal and the center electrode are placed. In operation, short circuits may arise since the liquid metal will grow or penetrate through the bore, and may form a bridge in the direction towards the ground or chassis electrode, thus short-circuiting the spark plug.
It has also been proposed to provide spark plugs which, in combination with their center electrode within the insulator, have a metal core made of copper or silver, and introduced into the insulator in the form of a powder or a rod. This core is heated and press-fitted within the insulator in order to obtain tight and close contact between the metal core and the insulator. Such spark plugs, also, require a considerable period of time before they reach the self-combustion temperature of deposits. Spark plugs of this kind are described, for example, in British Patent No. 547,119. It has also been proposed to construct spark plugs by introducing silver into the central bore of the insulator body by centrifugal casting, so that no gap will occur between the center core and the insulator (see U.S. Pat. No. 3,113,232). It has also been proposed to press-fit a metal core made of copper or nickel within the insulator, and to add materials to the metal core which modify its temperature coefficient of expansion so that it will match at least approximately that of the insulating body (see U.S. Pat. No. 3,061,756). Such spark plugs, also, due to the tight fit between the insulator and the spark plug, have good heat transmission characteristics so that, upon starting, or upon idle operation of the engine, the self-burning temperature is reached only late, if at all.
At high loading of the engine, and during extended operation under load, the spark plug may get very hot and, if the heat of the spark plug becomes excessive, the tip end of the insulator, or the metal core, may begin to glow, which may lead to misfires of the engine. Thus, the ideal spark plug should heat rapidly to a temperature which is above the self-igniting temperature of deposits, but will not heat to such an extent that glow ignition due to excessive heating of the insulator or surrounding elements will result. It is difficult to find materials which have the requisite characteristics of at least approximately matched temperature coefficients of expansion, which are insulating for the insulator, conductive for the center core, rapidly reach a temperature which is above the inherent combustion temperature of deposits, but will still have sufficient heat transfer characteristics to the metal housing so that the spark plug, even under extended heavy loading, will not reach the temperature at which glow ignition may result.